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SPARS
As is usually
the case with spar slots in foam wings, the slot was rather
oversized compared to the spar. This usually causes us to fill
the void with epoxy, adding unnecessary weight, just to get a
good attachment. The foaming expansion action of the
Polyurethane glue is tailor made to solve this problem.
The process
is done to one side of the wing at a time. The wing was placed
in one of the core shucks and a bead of Polyurethane glue was
placed in the bottom of the each slot. The spar was heavily
misted with water and placed into the slot. A strip of 1"
masking tape was then placed full length of the spar to confine
the expansion to the slot. A strip of waxed paper is then placed
on the wing and the top shuck placed atop that. This assembly is
then weighted down much as is done with an epoxy sheeting
process. I use steel weights that evenly distribute the load
over the assembly. The combined weight is approximately 150 lb.
Because I am in no hurry, I leave this to cure overnight.
Upon removing
all the weights, shucks, etc. I found that the glue had filled
the voids and even sent "runners" into the pores of the foam
itself for about 1/2 inch. This gave additional strength to the
bond of the spar to the foam.
SHEETING
Applying the
sheeting on this wing was made easier by the fact that it has
open bays with cap strips. This allows the sheeting to be
applied in smaller pieces than a fully sheeted wing. The
individual sheeted areas were applied one at a time. The
sheeting was positioned on the wing core and held in place by
masking tape. The tape was located such that it prevented the
expansion of the glue into areas it was not desired. The sheet
edge at the spar was taped full length to prevent the glue from
entering the bay area.
After
locating the sheeting it was folded back over the tape and
Polyurethane glue applied as noted earlier and spread to a very
thin film with a plastic spreader. Water was then misted on the
foam core where the sheeting would lie. As soon as this was done
the sheet was folded back into place, waxed paper separator
installed, top shuck installed and the assembly weighted as
noted earlier. Again, because I was not in a hurry, this was
allowed to cure overnight. There is probably some reduced amount
of time that could be allowed for the cure, but I will let that
be someone else's determination.
After the
large sheets were in place the cap strips were installed with
Gap-Filling CA that does not attack foam. The bays were then
lightened by "pocketing" out the individual bays on each side of
the wing to leave a 3/8 inch thick web in the middle of the
wing. The web left in the wing affords some amount of transverse
strength remaining in the wing while at the same time lightening
the wing and preventing the covering from contacting the foam
core. The "pocketing" was done with a soldering gun to which #12
or #14 copper wire from household "Romex" wiring was installed.
The wire was bent to the desired shape to just fit between the
cap strips to the desired depth and ledges bent in the wire to
ride on the cap strip. The cap strips were protected from the
heat of the wire with two layers of masking tape.
The remaining
leading edge and end caps were then applied and the wing sanded
for covering. If I were to have done a fully sheeted wing, I
would follow much the same process except for positioning the
sheeting on the core. I would likely have used masking tape in
selected locations to assure the sheeting remained properly
located. There is a limited amount of time allowed to spread the
film and water before the glue starts to react, but I had no
problems at all with the above. I believe any type sheeting
could be applied in the time allowed. It is doubtful that both
sides could be done in one setup. Besides, if you are in a hurry
you are in the wrong hobby anyway.
CAUTION: When
hot-wiring the bays of the wing it is advisable to provide
generous ventilation as melting polyurethane does give off some
toxic fumes. |